- Posted by: jtirenti
- Category: Piping
Most industrial plants have some underground piping systems, in other words located below the standard elevation of the ground defined in the plant. In most cases, these piping systems are installed during the construction of the new plant, and are rarely thought about until a problem arises.
An underground pipe is not the same as a buried pipe. Buried pipe is technically a subcategory of underground pipe. A buried pipe is defined as the pipe that is below ground level and in direct contact with it. Underground pipe is defined as pipe below the ground level that is not in direct contact with the soil. It is not accessed routinely and is normally located some distance from buildings and other plant facilities. It is usually contained within a concrete vault or trench.
Buried systems range from those with a minimal effect on the safe operation of the plant, such as storm drains, to safety-related systems such as auxiliary water supply pipes used to supply water to steam generators in some nuclear power plants if the main feed water system is not available. Other important systems that are often underground include fire protection, water and air service, electrical systems, instrumentation, as well as the emergency fuel line for a diesel generator.
Installation of a pipe section in a trench.
The integrity of a buried pipe must be evaluated for the range of anticipated loads. There are the following types of buried pipes:
- New or existing buried pipe, made of carbon steel or alloy steel, manufactured according to ASTM or API material specifications.
- Welded pipe joined by welding techniques from either the ASME code or API standards.
- Pipe designed, manufactured, inspected and tested in accordance with ASME B31.
Buried pipe may be subject to large bending and tensile loads due to the effects of soil movement. Sources of soil movement include differential ground settlement, fault displacement, displacement during earthquakes, frost heave or ice thawing.
Thawing settlement scenario.
The measurement or prediction of soil displacement requires a special experience. The non-linear behaviour of the pipe and soil are usually assessed with finite element analysis.
In order to avoid localized wrinkles or tensile fractures in circumferential welds, induced stresses must be kept under the yield stress of the steel. Appropriate deformation limits such as stress or curvature limits can be established based on detailed tests and analyses, taking into consideration the characteristics of this phenomenon. For these cases, it is not possible to develop simple design formulas for differential soil movements based on elastic stress analysis procedures.
A rigorous design and analysis involve a nonlinear pipe-soil interaction analysis. The model must consider the axial resistance of the pipe and the resistance to bending, the longitudinal resistance of the ground caused by adhesion and friction, and the transverse resistance of the soil. The resistance of the soil is generally idealized as an elastic spring.
The distributed soil resistance is modelled as a series of discrete springs that provide a specific resistance per unit length of the pipe.
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